A sound generator of this kind has, in a housing, at least one electroacoustic converter, preferably in the form of a loudspeaker. A membrane of the converter separates a rear volume from a front volume in the housing. The sound generator can be connected via a connection pipe to an exhaust gas-carrying exhaust gas line of the exhaust system, and the connection pipe then connects the front volume with the exhaust gas line fluidically and acoustically in the connected state.
Such a sound generator may be used, for example, as an active exhaust muffler to reduce undesired noises, which propagate as air-borne noise in the exhaust gas line. Corresponding active noise control is generated for this by means of the electroacoustic converter and emitted in a phase-shifted manner, so that sound and active noise control are superimposed to one another, which leads to a reduction of the amplitudes of the disturbing sound. In addition, or as an alternative, such a sound generator may also be used to specifically intensify or generate certain engine noises. The sound of an exhaust system or of the internal combustion engine can be specifically affected in this manner by means of such a sound generator.
The rear volume enclosed in the housing of the sound generator has a housing internal pressure, which must be essentially at equilibrium with an external pressure prevailing in the front volume when the membrane is not moving in order for the membrane to be able to assume its neutral central position. To prevent the membrane from performing permanent deflections from its neutral position, which can lead to damage to the membrane, in case of changes in weather, which are accompanied by a change in the ambient pressure, and during temperature changes, the housing may be equipped in the conventional manner with a pressure equalization opening, which makes possible a static pressure equalization between the rear volume and an area surrounding the housing. In order for such a pressure equalization opening to permit a static pressure equalization only, while it prevents a dynamic pressure equalization, such a pressure equalization opening is provided, as a rule, with a correspondingly small opening cross section. Static pressure equalization usually takes place at a frequency of less than 1 Hz. Contrary to this, a dynamic pressure equalization takes place, as a rule, at a frequency higher than 10 Hz. Dynamic pressure equalization must be avoided in order to make it possible to guarantee the ability of the converter to function.
To prevent splash water from entering the housing through the pressure equalization opening during the operation of the vehicle, it is possible to equip the pressure equalization opening with a corresponding splashproof protection. It is also conceivable to equip the pressure equalization opening with a semipermeable membrane, which is impermeable to liquids while it is permeable to gases. The permeability to gas of such a semipermeable membrane is selected to be such that the desired pressure equalization is possible.
In addition, the problem arises in case of all-terrain vehicles as well as of vehicles with off-road ability, especially in so-called SUVs, where SUV means sport utility vehicle, that the entire housing of the sound generator may be surrounded by water when driving through bodies of water. Even though the semipermeable membrane does offer protection against the entry of water into the housing in such cases as well, it is no longer able to ensure pressure equalization for the rear volume against the water pressure prevailing on the outside. In particular, the path of flow of ambient air into the rear volume is blocked by the water surrounding the housing. The temperature of the exhaust system rises markedly against the environment during normal operation of the vehicle. In particular, the temperature of the housing of the sound generator rises as well. The pressure rising in proportion to the temperature in the rear volume can be permanently equalized with the surrounding area by air leaving the rear volume in proportion to the temperature through the pressure equalization opening and entering the surrounding area. If the vehicle now passes through a body of water that is so deep that the housing will be flooded, the housing will cool down relatively sharply in a short time, and so will the air enclosed in the rear volume. Air would now have to enter the rear volume from the environment for pressure equalization. However, this path is blocked by the water that surrounds the housing. Thus, static pressure equalization is not possible in this special case. The position of the membrane changes greatly in the direction of the rear volume, and the membrane may become permanently damaged.
Furthermore, it may happen while driving through a body of water that water may enter the exhaust system through a tail pipe and reach the sound generator, as a result of which the latter is flooded quasi on the side of its front volume. The dynamic pressure of the water likewise pushes the membrane into the rear volume. A membrane drive of the converter cannot drive the membrane any more against this dynamic pressure or it can do so to a very limited extent only. Since the output of the usually electromagnetic membrane drive cannot be converted into motions of the membrane in this case, overheating of the membrane drive may occur.